Located around 523 km (325 miles) southeast of the Acropolis, the Arch of Hadrian, resembling in some respect a Roman triumphal arch, spanned an ancient road from the centre of Athens in Greece, to the complex of structures on the eastern side of the city that included the Temple of Zeus. It is said that the arch, commonly known as Hadrian’s Gate, was built to celebrate the arrival of the Roman emperor Hadrian in the city on the occasion of the dedication of the nearby temple complex in 131 or 132 AD and to thank him for his benefactions for the city.

In all probability, the arch was designed and built by an Athenian, but nobody is sure about that. As the arch has two inscriptions facing in opposite directions, naming both Theseus and Hadrian as founders of Athens, once it was maintained that the arch marked the line between the ancient part of Athens and the new regions of the city. However, that idea proved to be wrong by further excavation.

The design of the arch, made of Pentelic marble, is fully symmetrical from front to back and side to side. But the marble quarried from Mount Pentelic, 18`2 km northeast of the arch, is of a lower grade than that used in the other contemporary famous Athenian buildings. Nevertheless, the 59 feet (18m) high, 44 feet (13.5m) wide and 7`5 feet (2.3m) in depth Arch of Hadrian was constructed without cement or mortar from solid marble, using clamps to connect the cut stones. It has a 21 feet (6`5 m) single arched passageway at the lower level, supported by pilasters or rectangular columns and crowned with Corinthian capitals. While taller pilasters flank the outer corners of the lower level, the space between the outer pilasters and the arched opening was filled in with squared stones with drafted edges, probably to emphasize the design. The central passageway is flanked by a Corinthian column, erected on raise rectangular base, projecting from the centre of the wall.

The upper level of the arch is divided into three rectangular openings by a series of Corinthian columns and pilasters. The central opening of the upper level was originally closed off by a thin screen of stone and only the slots for its mounting are now preserved.

Arch of Hadrian has some basic differences from most of the preserved Roman Triumphal arches. While the Arch of Hadrian has only a single arched passageway at the lower level, the number of arched passageways of the lower level of the Roman triumphal arches was variable as was the presence of a secondary passageway along the long axis of the structure.

Roman triumphal arches also typically have a massive and solid upper level, often filled sculptural decoration, along with a dedicatory inscription. Apart from that, the Roman arches supported major stone or bronze statuary, often including a Quadriga, four-horse chariot or similar at top centre. However, although the design of the Arch of Hadrian has a very refined upper level, it does not allow the mounting of major decoration on top of the attic.

However, as it was common for this type of architectural form, some scholars think that there was a time when the top of the Hadrian's Arch was decorated with several statues, on either side of the central niche of the upper level. Based on the inscriptions, it is also estimated that Theseus and Hadrian were included among those statues. Nevertheless, it is also argued that there is no evidence of any type of doweling at the top of the lower level, which is needed to mount statuary. Furthermore, the stone on the upper surface is far too roughly worked to mount statues on that.